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Two University of Arizona researchers are adapting RTK-GPS-based auto guidance technology to push mechanical cultivation in cotton within 2.5 inches of the seed line to gain improved broadleaf control.

Pedro Andrade and Bill McCloskey kicked off their precision cultivation research project this spring at the Maricopa Agricultural Center. They are testing a real-time kinematic global positioning system (RTK-GPS) auto guidance system, the Trimble AgGPS NavController II, in a Case IH Puma Series tractor across a 2.5 acre cotton plot, with implements for bed listing, seed planting, and cultivation of seedlings three times in the season.

Andrade is an agricultural engineer and precision ag specialist; McCloskey is an Extension weed scientist at Tucson. Both have doctoral degrees from the University of California, Davis.

RTK-GPS is an ultra-precise version of GPS that allows sub-inch accuracy levels in positioning measurement.

Andrade says the technology combines two elements in positioning measurement. In addition to a satellite link to the rover (tractor), there is a radio communication link between the rover and a ground base station that carries positional correction data. Second, a patented algorithm solves the rover’s position mathematically with time and phase delay information of the signal.

“During bed formation and seed planting, the RTK-GPS system guides the tractor in a straight, pre-determined path,” says McCloskey. “Understanding the implement geometry allowed us to know exactly where the seed line is located.

“The cultivator is able to run closer to the seed line than with traditional cultivation, since the tractor knows within an inch the precise seed line location. This keeps the cultivator clear of the plants and enhances mechanical weed control efficacy.”

Monsanto’s Roundup Ready cotton became a hit with cotton growers in the 1990s, allowing growers to use Roundup herbicide in an over-the-top application to control broadleaf weeds and perennial/annual grasses.

The Arizona researchers are determining if RTK-GPS-guided mechanical cultivation can remove more weeds with steel than herbicide to hopefully save farmers’ money and benefit the environment.

“The less herbicide we apply, the less selection pressure there is for the development of herbicide-resistant weeds —what we don’t know is what are the trade-offs,” McCloskey notes.

“With the RTK-GPS system, the farmer would reduce herbicide use but increase diesel fuel use. We’re anxious to get additional project funding to see how this shakes out.”

In the test plot, the tractor pulled a 4-row, 14-foot cultivator manufactured by Bingham Brothers, Lubbock, Texas, in the 1990s.

The plot was planted April 8 with Deltapine 164 B2RF; mechanical cultivations were done May 15 (3- to 4-leaf stage), June 5 (about 10 leaves), and June 18 (20 to 22-inch cotton).

RTK-GPS systems cost in the $30,000 range and can come pre-installed on new tractors. McCloskey says total costs include the equipment installed on the tractor plus access to the base station network that enables radio communication with tractors to achieve sub-inch level accuracy.

The infrastructure of local towers has been built and funded by equipment dealers. Base station towers provide coverage to many locations, including the Yuma, Safford, and Central Arizona valleys.

It’s important for farmers to buy into this technology, McCloskey explains, but while RTK-GPS creates more accurate row listing, farmers want other perks for the price, including precise cultivation to reduce herbicide use.

“If you’re paying $25 per gallon for Roundup and broadcast spray 1 quart per acre, you’re spending $6.25 per acre for the herbicide,” he says.

“If we’re able to cultivate all but a 4-inch band on the seed line, we could reduce that to $1 to $2 per acre. If manufacturing costs of inputs are considered, I think precision cultivation will save cotton farmers money over herbicide use.”

Andrade says cotton is conventionally cultivated with two types of 6-row, 20-foot-wide cultivators: the Lilliston rolling cultivator or a combination of fixed-position disk openers and banana blades. Both systems cultivate 1.5 to 2 inches deep and within 4 to 5 inches of the seed line and utilize Alabama sweeps to move the soil back into place.

Some new cultivator systems on the market include electronic cameras or mechanical wands to gain closer seed line cultivation, but McCloskey says those systems don’t work well with cotton seedlings.

“I’ve worked with a mechanical wand-articulated electro-hydraulic quick-hitch guidance system, and the system limitation is that cotton needs to be about a foot tall with a woody stem for the mechanical wand to sense it. Otherwise the wand bends the seedling over. With the RTK-GPS system, we can safely cultivate seedling cotton.”

The RTK-GPS guidance system is also essential for spraying narrow bands of herbicide over the top so the spray band and the seed-line/uncultivated area are properly aligned.

The sky is the limit for RTK-GPS technology. Andrade says, “Enhanced cultivation with RTK-GPS technology has the potential to work with any crop grown in a bed.”

McCloskey’s goal in the trials is to save farmers money through decreased herbicide use, while improving overall weed control.

“There is a saying, ‘Steel is non-selective,’” Andrade says. “Weeds cannot build a resistance to cultivation; mechanical control is a simpler approach.”

In the Western U.S., more tillage is required for cotton than in other regions of the Cotton Belt since the crop is furrow irrigated. Two of the most common broad-leaf weeds in Arizona fields are Palmer amaranth and annual morningglory.”

Andrade and McCloskey are seeking funding for four more years of research, including the expansion of test acreage into larger commercial farmer fields, where the energy savings can be calculated more accurately.

“Determining the actual costs of the RTK-GPS-equipped tractor and the fuel consumption to pull the implement across large fields is how to get a handle on accurate energy savings,” Andrade says.

The researchers are seeking continued funding from the USDA’s Sustainable Agriculture Research Education program. The University of Arizona is providing the first-year funding.

One of the justifications for SARE funding would be to take the project “on the road,” McCloskey says.

During years 1 and 2, the system details would be worked out and the technology readied for field-scale testing. Years 3 through 5 would include field tests to collect crop budget information to demonstrate cost benefits and seek to work out answers to the concerns of the farmer cooperators.

While farmers across the Cotton Belt have reduced cotton acreage in ’08 due to higher grain prices, cotton remains the second largest acreage crop in Arizona.

According to the National Agricultural Statistics Service Arizona field office, about 141,000 acres of cotton was planted in 2008; compared to 170,500 acres harvested in 2007. Alfalfa ranks first, with 260,000-acres producing hay in 2008, up 10,000 acres from last year.

Andrade and McCloskey are convinced that RTK-GPS technology can improve standard cultivation and that farmers will embrace the system.

“We have a component of growers who are technologically innovative,” McCloskey says, “and I would hope after five years to have these early adopters buy into this technology — not just in cotton, but every row crop grown in Arizona. Their success will prompt others to accept the technology.”

The RTK-GPS technology requires a higher-educated work force that, over the long-term, could reduce labor needs, Andrade says, and a precision cultivation system works well in all soil types.

Cultivating so close to the seed line doesn’t damage the plants, if done properly, McCloskey says. The proof will come when cooperators harvest full-length strips of cotton.